The invention is related to the field of dyeing polymers, particularly the field of dyeing shape memory polymers (SMP), shape memory polymer surfaces and shape memory polymer films, and the use thereof for purposes of information coding and as a security feature.
Polymers can be coloured with dyes in a number of different ways. Commonly used methods include the mass dyeing, in which for example a pigment or dye is mixed with the polymer and the polymer is melted to enable the dye to infiltrate the polymer matrix. In other processes, the polymer is coloured by diffusing the dyes from a solution or dispersion, such as when dispersion dyes, for example basic dyes, acid dyes, metal complex dyes or reactive dyes are used to colour polymer fibres of polyester, polyacrylic nitrile, polyurethane, cellulose or polyamide, for example.
When reactive dyes are used, a covalent bond is formed between the dye and the polymer, which enables particularly good colour fastness to be achieved. For this, the dyes used, or their chromophores, must be sufficiently stable under the polymerisation conditions. A typical process for dyeing textiles is discontinuous dyeing (exhaust dyeing process), in which the fabric is passed through a dye bath containing dyes that have been dissolved or dispersed in water. In continuous dyeing (pad dyeing) the entire fabric is impregnated uniformly and coloured with dye evenly.
According to the intended use of a polymer, however, it may be desirable to mark a polymer item only on the surface or only on one side. In this regard, the previously known solutions for dyeing only the surface or one side of polymer items are only partly satisfactory, and still require considerable technical investment. Furthermore, it is difficult to adjust parameters such as the penetration depth of the dye into the respective polymer with conventional dyeing methods. It is therefore technically very challenging, if not impossible, to colour layers close to the surface in a targetted manner.
One option for surface marking or generally marking polymer item that is extremely adaptable in terms of application aspects consists in the use of an ablating laser beam. The surface can also be removed from polymer items selectively using mechanically abrasive processes and/or chemically by etching with acids or bases. The use of this form of marking offers particular advantages for shape memory polymers, particularly for labels or security marks of such, or for products made entirely of SMP. Since a mark applied to the SMP surface by means of non-dyeing laser engraving or the representation of information can be rendered temporarily invisible or hidden by appropriate programming of the SMP, security features can be introduced into the SMP that can help to increase security against counterfeiting of items, for example. Applications therefor are described for example in DE 10 2009 053 808.
The term shape memory polymer (SMP) is generally used to describe plastics that after being reshaped, are apparently able to “remember” their previous external shape, and effectively have a memory of their shape. In order to restore the former shape, the SMP must be exposed to a stimulus. This stimulus kann may be the application of heat, for example, by which the SMP in question is heated either directly or indirectly.
The SMP may be heated directly from the outside by hot air, IR radiation, for example by exposure to the sunlight or the airstream of a hot air blower, or by direct contact with a heat storage medium, such as a preheated fluid. Heat may be applied by immersing in warm water, for example.
According to other embodiments, the heat is supplied indirectly by an auxiliary material or filler material that is permanently embedded in the SMP material and heats the SMP matrix by interacting with an external electromagnetic field. Such auxiliary or filler materials may have a graphene structure, for example, such as exists in graphite, carbon nanotubes, graphene flakes or expanded graphite. Other particles with a nanoscale dimension can also be used as auxiliary materials and fillers. For example, magnetic nanoparticles, ferromagnetic particles, particularly NiZn particles, iron oxide particles and magnetite particles may be considered. Nanoclays may also be used as fillers. The nanoclays may be formed on bases of silicon nitride, silicon carbide, silica, zirconia and/or alumina, for example.
Other possible fillers are oligomeric silsesquioxanes, graphite particles, graphenes, carbon nanotubes, synthetic fibres, particularly carbon fibres, glass fibres or Kevlar fibres, and metal particles as well. Of course, combinations of these fillers may also be used. The fillers are suitable for adjusting the mechanical, electrical, magnetic and/or optical properties of an SMP and adapting it to its respective intended purpose.
One advantage of the auxiliary materials or fillers added to the SMP is that their size and material properties enable to them to absorb the energy of irradiated electromagnetic fields, convert it to heat and deliver it to the surrounding matrix of the shape memory polymer. In this way, an article made from SMP can be heated efficiently and change its shape rapidly without physical contact.
In particular, after marking on one side with graphic elements such as logos, glyphs, letters, numbers, symbols, QR codes, Data Matrix codes or barcodes, the contrast between the engraved and unengraved areas on articles made from polymers with shape memory is unsatisfactory, so that for example the automatic readout of codes with a reading device (scanner) is difficult, subject to errors or entirely impossible. However, it is precisely the easy readability of such representations combined with the shape memory properties of the polymers that can provide a preferred option for marking objects in a way that cannot be forged.
For these and other reasons, there is a need for the present invention.